BMI Explained and Its Limits

Body mass index divides your weight in kilograms by your height in metres squared. A 1.75 m adult at 75 kg scores 24.5, and the same frame at 95 kg scores 31. BMI is a screening number built from population data, not a diagnosis of anything. This page is general information, not medical advice. If you have a health condition, are pregnant, or are under 18, talk to a doctor or dietitian before changing anything based on a BMI reading.

What the formula calculates

The formula is weight in kilograms divided by height in metres squared. In imperial units it is 703 times weight in pounds divided by height in inches squared. Take a 68 kg adult at 1.62 m: 68 divided by 1.62 x 1.62 gives 25.9. Measure height without shoes against a wall with feet flat, and weigh on the same scale in the morning before eating, since a day of food and fluid can shift the reading by 1 to 2 kg.

The World Health Organization bands the score: under 18.5 is underweight, 18.5 to 24.9 is normal, 25 to 29.9 is overweight, and 30 and above is obese. Each band is a population marker that flags who may benefit from a closer look at waist size, blood pressure and blood glucose. None of those four cut points is a clinical finding on its own.

• Under 18.5 - Underweight band; a doctor may check thyroid function, digestion and eating behaviour behind it.
• 18.5 to 24.9 - Normal band used since the 1990s as the reference range for adults aged 20 to 65.
• 25 to 29.9 - Overweight band; risk rises further when waist passes 94 cm in men or 80 cm in women.
• 30 and above - Obesity band, split into class 1 (30 to 34.9), class 2 (35 to 39.9) and class 3 (40 plus).

Where the cut points came from

Belgian mathematician Adolphe Quetelet derived the ratio in the 1830s to describe the average build of conscript armies and whole populations, not to assess one person. Ancel Keys renamed it body mass index in a 1972 paper comparing skinfold measurements across 12 population samples. The World Health Organization adopted the current bands in the 1990s from mortality curves drawn out of large cohorts and life insurance tables.

Those curves came mostly from European and North American adults and ran into the hundreds of thousands of people, but the shape of the curve describes groups. A single reading sits on that curve with a wide margin of error, roughly plus or minus 3 to 4 points for an individual.

What BMI cannot see

Five kilograms of muscle and five kilograms of fat weigh exactly the same, and the formula counts them identically. A 1.8 m rugby forward at 100 kg scores 30.9, the same as a sedentary adult of that height carrying 35 percent body fat. A DEXA scan, hydrostatic weighing or a trained caliper test can separate the two; a bathroom scale and a tape measure cannot.

Fat placement changes risk at the same score. Visceral fat wrapped around the liver and pancreas drives insulin resistance harder than fat stored under the skin of the hips and thighs. A waist above 94 cm for men and 80 cm for women marks raised risk under International Diabetes Federation thresholds, and that gap can sit inside a BMI of 24.

Why ethnic group and age shift the bands

A 2004 WHO expert consultation recommended lower action points for adults of South and East Asian descent: overweight at 23 and obesity at 27.5. At the same BMI, South Asian adults average 3 to 5 percent more body fat than European adults, and more of it sits deep in the abdomen. For a 1.65 m adult, a BMI of 23 works out at about 63 kg.

Muscle mass falls roughly 3 to 8 percent per decade after age 30 and faster after 60 unless the person trains against it. A 70-year-old at BMI 26 may therefore carry less muscle and more fat than a 30-year-old with the same score. Children and teenagers need age and sex percentile charts rather than adult cut points, because normal body composition shifts every year of growth.

Reading a BMI without misreading it

Treat the score as one input beside a waist-to-height ratio, resting heart rate and a strength benchmark such as how many full push-ups you can complete. Track the trend across 3 months instead of reacting to one morning, since water, salt and glycogen can move weight 1 to 2 kg in 24 hours.

A second measurement plus a 3-month trend catches the cases a lone score gets wrong.

If a BMI reading or waist measurement worries you, the useful next step is a doctor or dietitian who can order blood work and look at the whole picture. Disordered eating is a real risk when numbers start driving food rules; in Australia the Butterfly Foundation helpline runs on 1800 33 4673, and in the US you can call or text 988 for crisis support.

• Waist-to-height ratio - Divide waist in cm by height in cm; 0.5 or less is the common target for adults under 65.
• Waist circumference - Measure at the top of the hip bone after a normal exhale, without pulling the tape tight.
• Body fat percentage - Measured by DEXA, calipers or a bioimpedance scale, each carrying 3 to 5 percent error.
• Strength benchmark - Full push-ups or a goblet squat weight logged monthly shows lean mass better than a scale.